The invention relates to fuel cells. The invention also relates to determining equivalent series resistance of fuel cells and to shunting fuel cells.
Fuel cells are well known in the art. A fuel cell is an electrochemical device which reacts a fuel and an oxidant to produce electricity and water. A typical fuel supplied to a fuel cell is hydrogen, and a typical oxidant supplied to a fuel cell is oxygen (or ambient air). Other fuels or oxidants can be employed depending upon the operational conditions.
The basic process in a fuel cell is highly efficient, and for those fuel cells fueled directly by hydrogen, pollution free. Further, since fuel cells can be assembled into stacks of various sizes, power systems have been developed to produce a wide range of electrical power outputs and thus can be employed in numerous commercial applications. The teachings of the following patents, U.S. Pat. Nos. 4,599,282; 4,590,135; 4,599,282; 4,689,280; 5,242,764; 5,858,569; 5,981,098; 6,013,386; 6,017,648; 6,030,718; 6,040,072; 6,040,076; 6,096,449; 6,132,895; 6,171,720; 6,207,308; 6,218,039; and 6,261,710 are incorporated by reference herein.
A fuel cell produces an electromotive force by reacting fuel and oxygen at respective electrode interfaces which share a common electrolyte.
In a fuel cell, fuel such as hydrogen gas is introduced at a first electrode (anode) where it reacts electrochemically in the presence of a catalyst to produce electrons and protons. The electrons are circulated from the first electrode to a second electrode (cathode) through an electrical circuit which couples these respective electrodes. Further, the protons pass through an electrolyte to the second electrode (cathode). Simultaneously, an oxidant, such as oxygen gas, (or air), is introduced to the second electrode where the oxidant reacts electrochemically in the presence of the catalyst and is combined with the electrons from the electrical circuit and the protons (having come across the electrolyte) thus forming water. This reaction further completes the electrical circuit.
The following half cell reactions take place:
H2xe2x86x922H++2e-xe2x80x83xe2x80x83(1)
(xc2xd)O2+2H++2e-xe2x86x92H2Oxe2x80x83xe2x80x83(2)
As noted above the fuel-side electrode is the anode, and the oxygen-side electrode is the cathode. The external electric circuit conveys the generated electrical current and can thus extract electrical power from the cell. The overall fuel cell reaction produces electrical energy which is the sum of the separate half cell reactions occurring in the fuel cell less its internal losses.
Although the fundamental electrochemical processes involved in all fuel cells are well understood, engineering solutions have proved elusive for making certain fuel cell types reliable, and for others economical. In the case of polymer electrolyte membrane (PEM) fuel cell power systems reliability has not been the driving concern to date, but rather the installed cost per watt of generation capacity has. More recently, and in order to further lower the PEM fuel cell cost per watt, much attention has been directed to increasing the power output of same. Historically, this has resulted in additional sophisticated balance-of-plant systems which are necessary to optimize and maintain high PEM fuel cell power output. A consequence of highly complex balance-of-plant systems is that they do not readily scale down to low capacity applications. Consequently, cost, efficiency, reliability and maintenance expenses are all adversely effected in low generation applications.
It is known that PEM fuel cells can operate at higher power output levels when supplemental humidification is made available to the proton exchange membrane (electrolyte). In this regard, humidification lowers the resistance of proton exchange membranes to proton flow. To achieve this increased humidification, supplemental water can be introduced into the hydrogen or oxygen streams by various methods, or more directly to the proton exchange membrane by means of the physical phenomenon known as of wicking, for example. The focus of investigations, however, in recent years has been to develop membrane electrode assemblies (MEA) with increasingly improved power output when running without supplemental humidification. Being able to run an MEA when it is self-humidified is advantageous because it decreases the complexity of the balance-of-plant with its associated costs. However, self-humidification heretofore has resulted in fuel cells running at lower current densities and thus, in turn, has resulted in more of these assemblies being required in order to generate a given amount of power.
While PEM fuel cells of various designs have operated with varying degrees of success, they have also had shortcomings which have detracted from their usefulness. For example, PEM fuel cell power systems typically have a number of individual fuel cells which are serially electrically connected (stacked) together so that the power system can have a increased output voltage. In this arrangement, if one of the fuel cells in the stack fails, it no longer contributes voltage and power. One of the more common failures of such PEM fuel cell power systems is where a membrane electrode assembly (MEA) becomes less hydrated than other MEAs in the same fuel cell stack. This loss of membrane hydration increases the electrical resistance of the effected fuel cell, and thus results in more waste heat being generated. In turn, this additional heat drys out the membrane electrode assembly. This situation creates a negative hydration spiral. The continual overheating of the fuel cell can eventually cause the polarity of the effected fuel cell to reverse such that it now begins to dissipate electrical power from the rest of the fuel cells in the stack. If this condition is not rectified, excessive heat generated by the failing fuel cell will cause the membrane electrode assembly to perforate and thereby leak hydrogen. When this perforation occurs the fuel cell stack must be completely disassembled and repaired. Depending upon the design of fuel cell stack being employed, this repair or replacement may be a costly, and time consuming endeavor.
Some of these problems are solved by fuel cell systems including removable modules as described in commonly assigned patents. For example, commonly assigned U.S. Pat. No. 6,218,035 to Fuglevand et al., incorporated herein by reference, discloses a proton exchange membrane fuel cell power system including a plurality of discrete fuel cell modules having multiple membrane electrode diffusion assemblies. Each of the membrane electrode diffusion assemblies have opposite anode and cathode sides. Current collectors are individually disposed in juxtaposed ohmic electrical contact with opposite anode and cathode sides of each of the membrane electrode diffusion assemblies. Individual force application assemblies apply a given force to the current collectors and the individual membrane electrode diffusion assemblies. The proton exchange membrane fuel cell power system also includes an enclosure mounting a plurality of subracks which receive the discrete fuel cell modules. In such a modular design, if one fuel module fails, it can be removed and replaced without the difficulty of disassembling a stack.
Attention is directed to U.S. Pat. No. 6,096,449 to Fuglevand et al., which relates to the humidification problem, and which is incorporated by reference herein. This patent discloses a shunt controller which is electrically coupled with a fuel cell and which, at times, shunts electrical current between the anode and cathode of the fuel cell. The controller comprises voltage and current sensors which are disposed in voltage and current sensing relation relative to the electrical power output of the fuel cell. The controller, under certain circumstances or at times (e.g., if voltage or current output of the fuel cell is below a predetermined minimum), closes an electrical switch to shunt current between the anode and the cathode of the fuel cell. Substantially simultaneously, the controller causes a valve to terminate the supply of fuel gas to the fuel cell. Alternatively, the shunt controller periodically shorts current between the anode and cathode of the fuel cell, while simultaneously allowing substantially continuous delivery of fuel gas to the fuel cell. The periodic shorting increases the overall electrical power output of the fuel cell. It is speculated that this repeated, and periodic shorting causes each of the fuel cells to be xe2x80x9cconditionedxe2x80x9d, that is, such shorting is believed to cause an increase in the amount of water that is made available to the MEA of the fuel cell thereby increasing the MEAs performance. It is also conceivable that the shorting provides a short term increase in heat dissipation that is sufficient to evaporate excess water from the diffuser layers which are mounted on the MEA. This evaporation of water thus makes more oxygen from the ambient air available to the cathode side of the MEA. Whatever the cause, the shorting appears to increase the proton conductivity of the MEA. This increase in proton conductivity results in a momentary increase in the power output of the fuel cell which diminishes slowly over time. The overall increase in the electrical power output of the fuel cell, as controlled by the adjustably sequential and periodic shorting of individual, and groups of fuel cells, results in the entire serially connected group of fuel cells to increase in its overall power production.
All circuits, including circuits that appear in circuit schematics to be mostly capacitive or inductive circuits, or that do not even contain resistors, still posses some value of resistance. This resistance is referred to as ESR (Equivalent Series Resistance), and an ESR value is typically quoted for capacitors.
While ESR of a fuel cell can easily be measured on a test bench, in situ measurement of resistance a fuel cell is not convenient. ESR could be measured in fuel cells using high frequency AC techniques, by applying an AC current through a fuel cell membrane and measuring voltage across the membrane. However, high frequency equipment is very expensive and therefore not economically included in fuel cell installations. Further, an AC power source is not always available, particularly with a fuel cell used in a DC application.
It is known that there is a relationship between ESR of a fuel cell and level of hydration of a fuel cell. This is described, for example in an article by P. D. Beattie et al titled xe2x80x9cIonic Conductivity of Proton Exchange Membranes,xe2x80x9d Journal of Electroanalytical Chemistry, Volume 503 (2001), pp. 45-56.
Therefore, it would be useful to be able to be able to economically measure ESR of a fuel cell, in situ, in an operating fuel cell system without having to use AC high frequency measurement equipment.